How Do Pet Scans Work? Physics.

Credit: Health and Human Services Department, National Institutes of Health, National Institute on Aging.

If you’re a sucker for medical shows on tv, you have no doubt heard the term PET scan before. A doctor will mention it, then the patient is whisked away to some big, fancy looking machine that miraculously diagnoses the patient and saves them from a scary disease!

But what is actually happening in that weird donut-shaped box?

Positron Emission Tomography (PET) is a nuclear medical imaging test that looks for a variety of diseases in your body using radioactive substances. The most commonly used substance in PET scans is the fluorine-18 isotope, as it decays rather quickly (half life of approximately 20 minutes). The isotope is implanted into a simple sugar molecule like glucose (called fluorodeoxyglucose or FDG for short), which is injected into your bloodstream. In the presence of a disease, the sugar is metabolized more quickly, which can be picked up by the scanner. The computer can then convert the signal it receives from the scanner into detailed 3D images, which shows how the tissue and organs are performing. This particular kind of PET scan is most commonly used to look for cancers since they thrive on sugar.

How Do They Work?

Once the sugary substance is injected into the bloodstream, it travels to your organs where the sugar is metabolized. The higher the metabolic rate of that particular organ, the more sugar you expect to see in that area. Since diseases and tumours have a higher metabolic rate than our organs do, there will be a larger amount of sugar wherever the abnormality is.

Unfortunately, we can’t just “see” the sugar. We need a radioactive substance or “tracer” to help us see this effect. This tracer is what allows us to see the abundance of sugar molecules within the body. The tracer undergoes a process known as positive beta decay - where a proton inside the nucleus of the tracer is converted into a neutron, while releasing a positron (the antiparticle of the electron) and another particle known as an electron neutrino. This can be shown in the following way:

proton → neutron + positron + other particle

This positron then travels a very short distance (1nm, or half the width of a strand of hair), before it loses most of its kinetic energy. At this point, it has slowed down enough to interact with an electron and together, they annihilate one another. All of the energy from this annihilation is converted into what is known as gamma radiation (high energy photons). The interesting thing about this particular emission of gamma rays is that by obeying the law of conservation of momentum, the two gamma rays are ejected at 180 degrees apart. These gamma ray pairs are then detected and converted into a signal, eventually used to map your organs and tissue, as seen in the picture below!

Credit: HyperPhysics

It should be noted that the photons which are not detected in pairs (i.e. not detected within a window of a couple of nanoseconds at 180 degrees apart) are ignored.

How Do They Differ From Non-Nuclear Scans?

The main difference between nuclear and non-nuclear medicine imaging stems from the where the ionising radiation occurs. For example, X-ray and computed tomography (CT) images are formed when X-rays pass through your body and are measured on the other side, where your body creates a “shadow”. A PET scan however, is formed from the radiation coming from inside your body. This makes you temporarily radioactive, unlike the previous scans. X-rays and gamma rays have similar properties, but arise from different parts of the atom (gamma rays = nucleus of an atom, X-rays = electron shell). However, since these gamma rays come from within us, they are often better at detecting changes in our tissue early on, making it a more accurate way to check for fast spreading diseases.

But why choose between one or the other? It is quite common to use multiple imaging techniques together to get a better image of the damaged tissue. PET scanners are commonly combined with CT scanners (known as PET-CT scanners) to create detailed images of slices of the inside of your body. From here, doctors can then precisely locate any abnormalities in the body which allows for a more accurate diagnosis.

Credit: University of Pittsburgh Medical Center.

The Downsides

Unfortunately, all good things come at a price, and PET scans are no exception. Depending on the location of the body scanned, it can cost anywhere between $2000 - $20,000 USD! And although the dosage of radioactive material injected into your bloodstream is not dangerous, patients are often administered other drugs as part of the scan, which may come with some side effects. Unfortunately, false positives also occur in people with diabetes. And for all of you claustrophobes out there, sitting in a tiny machine for around 30 minutes isn’t necessarily pleasant.

Applications

So what can a PET scan tell you? In terms of cancerous growths, it can:

Locate the site of the cancer.

Determine the size of the tumor.

Differentiate benign from malignant growths.

Discover if the cancer has spread.

Monitor the success of therapy.

Detect any recurrent tumors.

Along with cancer, it is very useful in helping detect diseases such as:

Alzheimer’s disease or dementia.

Epilepsy.

Movement disorders (such as Parkinson’s disease).

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